Ask people to describe themselves politically, and many will choose the label “moderate” when the alternative is the stigmatized word “extreme.” But ask moderates if they are angry that billionaires have had tax cuts while millions of people don’t have health insurance, and you will find many are—often extremely so.

Too much polling is designed to produce an answer that suits an argument. Questions are loaded. Samples are small. That’s why the analysis of actual electoral experience is so important.

With that in mind, those grappling with the argument that Bernie Sanders is too left-wing to beat Donald Trump might like to cast their eyes across the Atlantic and consider Labour’s unexpected success in denying the Tories a majority in the United Kingdom’s 2017 general election.

When Jeremy Corbyn became Labour leader in 2015, his opponents argued relentlessly that he would be an electoral disaster. That claim was the main pretext for Labour MPs to force a second leadership election a year later. Corbyn won again comfortably, but the attempt to oust him did the party no favors: Labour’s position in the polls slumped to about 25 percent.

The Tories saw their chance, as insiders have now revealed, to finish off the Labour Party for the next 20 years. Riding high in the polls, Prime Minister Theresa May called a snap election in April 2017. Most pundits thought she would win by a landslide.

As an adviser to the Labour leader, I was a member of the party’s strategy group, which included long-standing officials opposed to Corbyn. They told us that “campaigns don’t move opinion more than 2 or 3 percent,” that “young people don’t vote,” and that his policies wouldn’t have an impact because “no one reads manifestos.” Labour’s pollsters predicted the party would lose nearly half the 232 constituencies we held at the time.

In eight weeks, the Corbyn campaign proved them totally wrong. The Labour vote increased from 9.3 million (30.4 percent) in 2015 to 12.9 million (40 percent)—our best vote since 1997 and the biggest increase in our vote share from one election to another since 1945.

In making a net gain of 30 seats, Labour denied the Tories a parliamentary majority—and forced May to abandon plans to escalate austerity. In producing a hung Parliament, the election transformed the voting arithmetic around Brexit, which May finally realized in opening talks with Corbyn a few weeks ago.

Corbyn’s success shocked his critics, but the outcome was no surprise to his team. A revolt against the political establishment—which had been locked in a neoliberal consensus since the Thatcher era—had been brewing well before voters rejected the advice of all the major parties in the 2016 EU referendum. The 2015 general election already saw disillusionment with the three main Westminster parties reach the point that they could muster the votes of only less than half the electorate between them. The other 23 million or so registered voters either abstained or supported smaller parties.

In 2017, Corbyn succeeded in winning back many voters who had migrated to other parties and in attracting new ones; the campaign saw a 51 percent increase (compared with 2015) in 18-to-24-year-olds registering to vote.

Labour’s manifesto, which was in some respects more radical than Sanders’s platform, was frequently described as the star of the show. Far from not being read, the hard copy became a collector’s item, and Labour’s website had 5.3 million new-user sessions in seven weeks.

A British parliamentary election and a US presidential one are, of course, very different events. But there are parallels, too, not least in the emergence of aggressively “populist” variants of neoliberalism in both countries.

If this politics is to be defeated, it will not be through a revamping of the Clinton-Blair formula of Chicago School economics, neocon foreign policy, and social liberalism. That brand has a diminishing following even in affluent cosmopolitan centers—never mind the electorally crucial rust belts of the two countries.

Trump’s administration has actually been drawn from political insiders and the corporate elite. But he will try to deflect from the squalid reality of his presidency by using special counsel Robert Mueller’s investigation to play victim. He will portray any moves against him in Congress—however justified they may be—as a vendetta by the political establishment.

This “Washington outsider” positioning will be much easier for him to sustain if his opponent is seen to be an insider—particularly one who campaigns on a similar narrative to Hillary Clinton, as Joe Biden appears to be doing.

The Democrats will not win by piling up votes in California and New York. We need a candidate with the politics and authenticity to enthuse alienated voters in the states that will decide the outcome. Sanders did that in 2016, and the midterms demonstrated we can win on a bold progressive agenda. There is no reason at all to believe that strategy has yet reached its full potential.